1. Field of the Invention
The invention relates to a method producing a doped zone of one conductivity type in a semiconductor body and somewhat more particularly to a method of producing such a zone which eliminates the "emitter-dip effect" within the semiconductor body.
2. Prior Art
In the design of high frequency transistors, it is desirable to maintain the maximum oscillation frequency as high as possible. This feature is generally equivalent to a high cut-off frequency at a low base resistance. Further, high frequency transistors generally require as thin a base zone as possible.
Canadian Pat. No. 878,173 (which corresponds to Austrian Pat. No. 313,981) suggests that high frequency transistors with suitable characteristics can be produced by continuing the emitter diffusion until the emitter diffusion front begins to advance or push the base diffusion front into the collector region which exists at the end of base diffusion region. In accordance with this prior art teaching, the base diffusion front is thus prevented from moving onward with increased emitter diffusion (the so-called "emitter-dip effect").
Such advancement of the base diffusion front may be explained, for example, in situations where an emitter zone has an unusually high dopant (i.e. phosphorus) concentration in the surface region of a silicon semiconductor body, since such high dopant concentration causes lattice distortion, crystal faults, phosphorus precipitation and the like to occur and expand in the region of the base zone lying beneath the emitter zone so as to cause an increased diffusion coefficient. This in turn causes the region of the base zone lying beneath the emitter zone to buckle out and during the necessary subsequent deepening of the emitter zone, to project out in relation to the emitter zone. The diffusion profile of the emitter zone must therefor be made deeper than would be necessary without the so-called "emitter-dip effect." As a result, an undesirable high inner base resistance occurs in the region below the emitter.
The heretofore noted Canadian patent does, in fact, provide some solution to the described problems. However, on the one hand, the emitter-dip effect is not avoided by following the teachings of this prior art since diffusion is only continued until the start of the emitter-dip effect and on the other hand, it is difficult with this process to determine or control the time at which the emitter-dip effect begins, particularly during mass production.
M. Takagi et al in "Supplement to the Journal of the Japan Society of Applied Physics," Vol. 42, 1973, pages 101-109, suggests a procedure for depositing a polycrystalline semiconductor layer highly doped with arsenic or phosphorus onto a semiconductor body whereby such doped polycrystalline layer acts as a diffusion source for the underlying body. This process, which allegedly is suitable for mass production and for the production of high dopant concentrations at a surface of a semiconductor body, does not eliminate the "emitter-dip effect" so that the devices produced thereby will still exhibit the disadvantages associated with such emitter-dip effect.